schema: add armv6l as valid arch for linux (Raspberry Pi 1)

[alban]

Some arm architectures are already added by #304 for #284. But according to rkt/rkt#730 (comment) this is not enough for Raspberry Pi 1:

@alban, if someone would like to use rkt on a Raspberry Pi 1 (there are about 5 million devices out there), the minimum supported ARM version should be ARMv6. This is also the default when building a GOLANG binary with GOARCH=arm. For this reason I strongly recommend to support armv6l too! BTW, a armv6l binary is compatible and runs absolute fine on a `armv7l´ machine - but not the other way around.

/cc @DieterReuter

The current support is:

var ValidOSArch = map[string][]string{
        "linux":   {"amd64", "i386", "aarch64", "armv7l", "armv7b"},
        "freebsd": {"amd64", "i386", "arm"},
        "darwin":  {"x86_64", "i386"},
}

[alban]

@DieterReuter : would you like to do write PR for this? Do you have the hardware to test?

https://github.com/appc/spec/blob/master/schema/types/labels.go#L9

[DieterReuter]

@alban, I do have the hardware available and will prepare a PR. Give me just a few days, please!

[jonboulle]

@DieterReuter how goes?